1. Field of the Invention
The present invention relates to a process for producing the center electrode of a spark plug in an internal combustion engine which has a layer of noble metal provided on the tip of the electrode, particularly in the periphery of its cylindrical tip.
2. Prior Art
The electrodes of a spark plug for use in an internal combustion engine desirably have a small diameter which is effective for reducing the discharge voltage and improving the ignition performance due to unhindered flame development. However, a thin electrode is easily heated up and dissipates less heat, and if, as in the usual case, it is made of a less noble metal such as a common nickel alloy, the electrode wears rapidly at elevated temperatures. In order to avoid this problem, several methods have been proposed for providing the tip of the center electrode with a noble metal layer having improved resistance to corrosion and spark wear. In one method, a noble metal is bonded to the tip of a center electrode and the assembly is extruded through a die to form a layer of the noble metal that covers the entire part of the electrode tip (see Japanese Patent Publication No. 22989/1974). According to another method, a pipe or ring of a noble metal is bonded to the tip of a center electrode (see Japanese Patent Public Disclosure No. 163976/1982). The third method proposes the use of a wire consisting of a noble metal cladding and a less noble metal core, said wire being cut to a given length suitable for bonding to the tip of a center electrode. However, these methods are expensive and require the use of costly noble metals not only in the area that must be protected from corrosion and wear but also in other areas that need no such protection.